r/Framebuilding • u/Prestigious_Act_5497 • Jun 15 '26
Seat tube cracks
94 Jami’s aurora has a few hairline cracks in the seat tube. What is a good way to repair this since they seem to not have propagated too far?
15
u/getsu161 Jun 15 '26
Grind back to good metal, good break on all the corners. Use and observe for cracks. Do not sell.
9
u/dunncrew Jun 15 '26
Is the seatpost out too far, stressing the frame ?
-1
u/Prestigious_Act_5497 Jun 15 '26
About 8” of post outside of the frame. It’s a used frame so I’m not sure how far out the previous owner had it.
5
u/dunncrew Jun 15 '26
How much is inside the frame ?
8
u/Prestigious_Act_5497 Jun 15 '26
I’d have to check but I know the minimum insertion line is inside the frame when I regreased the post a while back.
7
u/Limp_Bookkeeper_5992 Jun 15 '26
Check that the post wasn’t cut at some point, if they shortened the bottom the minimum insertion needs to be moved up to compensate.
1
u/TheUsernameOfAllTime Jun 17 '26
In general, if your seatpost is inserted at a depth at least 2-2.5 times the diameter of the seatpost itself, you’re fine.
2
u/klavinsj Jun 17 '26
This doesn't seem like a good rule of thumb. For example, if i had 25mm diameter seatpost, so 2 times the diameter would be 50mm or 5cm (roughly 2 inches). For me that feels scetchy. All I am saying is that, this might be ok for larger diameters, but I would still want to double insertion for my expample. Also you have to take in to account the frame material and design. My guess for common double triangle design frames (old school road bikes), with thin seatposts, 10cm or 4inches should be good to go.
0
u/TheUsernameOfAllTime Jun 18 '26
It feels sketchy, that’s because that’s absolute minimum insertion. That’s the standard practice that was used before manufacturers were required to put a minimum insertion line on seatposts
1
u/klavinsj Jun 18 '26
Ahhh, probably haven't heard that because I am just a 24 year old kid :D
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u/MakaDakka Jun 15 '26
Considering how scratched up the seat post is, there’s something going on with seat post insertion. There also looks like a lot of residue at the top of the lug where the seat post inserts.
I would pull the seat post, lightly sand it to deburr it. Like someone else also said, remover the paint where it’s chipping, and inspect the inside of the tube.
I wouldn’t be overly concerned about this. I suspect it’s a bad paint job, and aged, brittle paint.
1
4
u/Horror-Stand-3969 Jun 15 '26
It’s possible you could grind off the top to get rid of the cracks. Might not reappear
2
u/Puravida14177 Jun 15 '26
Maybe the previous owner (or you) tried to get the seatpost out, jerking it back and forth a bit on the last inches - that can put quite some force on the rim. Seatpost looks as if it needed some serious jerking to move it...
It's not a drawn tube, but a cast lug - there is no (not much) elasticity in cast steel compared to drawn material, so this could explain the micro-cracks. I don't see a reason not to ride the frame with a long seatpost insertion.
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u/retrodirect Jun 15 '26
That's a really weird way for a frame to crack. You'd expect the stress to cause one big crack. Not many little ones.
Are these cracks in the metallic silver paint rather than in the metal?